How a warmer Arctic could lead to colder winters in Britain

When you’re chipping ice from your car
windscreen or sheltering from the snow on a windswept platform, you
might find yourself wondering how climate change fits in with the
flurry of cold winters we’ve seen in recent years.

One theory suggests very cold winters in the
northern hemisphere could be linked to rapidly increasing
temperatures much further north – in the Arctic. A
new paper outlines three ways scientists
think the two could be linked.

Rapid Arctic warming

Temperatures in the Arctic have been increasing
almost
twice as fast as the global average. This is
known as
Arctic amplification. As Arctic sea-ice
shrinks, energy from the sun that would have been reflected away is
instead absorbed by the ocean.

From 2007 to 2013, we’ve seen some of the lowest
summer sea ice levels since records began, as shown in the graph
below. This has coincided with a number of extreme cold and wet
events across the mid-latitudes of the northern hemisphere,
including the
cold start to 2013 in the UK and the record low
temperatures in the US and Canada during the 2013-14
winter.

Some scientists have suggested this extreme
weather is a result of Arctic amplification. A new paper in
published in Nature Geoscience runs through three potential
explanations of what could be happening.

Storm tracks

The wintry weather we’re used to in the UK is
largely caused by storms coming in from the mid-Atlantic. The
passage these storms take are known as ‘storm tracks’.

When the storm tracks fall directly over the UK,
we get wet and mild conditions. When the storm tracks pass to the
south, colder air is drawn in from northern Europe and Russia,
making winters much colder.

The position of storm tracks is affected by a
natural fluctuation known as the North Atlantic Oscillation (NAO).
When the NAO is positive, the storm tracks fall over the UK; when
negative, they fall to the south. So a negative NAO is associated
with colder winters – like the ones we’ve seen recently.

The NAO has been largely negative since the turn
of the century and some scientists suggest it is being affected by
recent changes in sea ice. The authors say in the paper:

“Observational analyses
have shown significant correlation between reduced Arctic sea-ice
cover and the negative phase of the winter NAO.”

But they also say that the NAO is
naturally highly variable, and so the impact of reduced sea-ice is
only small in comparison. This suggests that variations in the NAO
are still predominantly driven by its natural
fluctuations.

The NAO has been mainly negative since the turn of
the century, as shown by the blue lines on the graph above. Source:NOAA

Jet stream

Scientists also think Arctic amplification could
be affecting the
jet stream – a band of fast-flowing air high
up in the atmosphere.

The jet stream received a lot of media attention
in recent UK winters as a possible cause of prolonged cold weather.
It’s driven by the temperature difference between the Arctic and
the mid-latitudes.

The theory goes that as the Arctic warms, this
temperature difference decreases. This means the jet stream
weakens, causing it to meander more and allow cold air to pulled
eastwards over the UK.

While some initial observations support the
theory, there is not currently enough evidence to be convincing.
This leads the authors to say that:

“[C]hallenges remain in
linking Arctic amplification directly to changes in the speed and
structure of the jet stream.”

Rossby waves

The third explanation involves ‘Rossby waves’ – which
consist of large air masses at high-altitude. Rossby waves move
north and south and create meanders in the jet stream. These waves
can slow down or ‘block’, at which point the weather experienced on
the ground will remain for some time. This could be a prolonged
period of cold or rain, or a heatwave in the summer.

Reductions in sea-ice extent in the Arctic have
been linked to high pressure systems over the Arctic that push cold
air towards northern Europe and brings cold weather to the UK. So
these high pressure air masses may have a role in locking weather
conditions in. But the paper highlights that modelling studies have
not been able to simulate these wave changes particularly
well.

This is an active area of research.
Another
recent paper, published this week in Proceedings
of the National Academy of Sciences, the impact of Arctic
amplification on Rossby waves is linked to a range of extreme
weather events in northern hemisphere summers, such as Europe’s
heatwave in 2003.

Knowledge gaps

With three potential links between Arctic
amplification and extreme mid-latitude weather, there is still a
long way to go before any relationship is fully
understood.

There is relatively little data around the poles
and Arctic amplification has only been detected in the last two
decades.

However, the paper stresses that the question is
“a critical one” as scientists predict Arctic amplification will
continue into the coming decades.

The paper also serves as a reminder that
increasing global temperatures doesn’t preclude periods of very
cold weather. Indeed, the paper concludes that:

“Cold winters such as
the one experienced in 2013-2014 have occurred before and are
expected as part of normal weather variability even on a warmer
planet.”